`meta_background_content_paint_content` was mixing two different
coordinate systems in `actor_pixel_rect`. It was initialized with
actor-local coordinates and then `if (self->clip_region)` would be
treated as stage coordinates. This worked because `self->clip_region`
was only non-NULL outside of the overview where both coordinate systems
were the same. So it always got the right answer, possibly by accident.
In order to enhance the function however we will need to know which
coordinate system we're working in, so now we make it explicit.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1363
30809665d8